A voltage boost circuit or charge pump is an electronic circuit that uses capacitors for energy storage to create a higher voltage power source. By way of one implementation, voltage boost circuits are needed for eDram's boosted word line VPP, and negative word line off voltage, VWL.
A challenge with charge pumps is that when creating a higher voltage power source, e.g., two or three times a supply voltage, voltages may be generated in excess of the oxide-stress limit of a field effect transistor (FET), i.e., a stress limit of the gate oxide thickness. That is, node voltages within a voltage boost circuit can exceed the reliability limits of maximum Vdd voltage. This may cause a failure of the charge pump and hence fail to provide the required voltage boost.
To overcome these reliability issues, FET devices are designed to have an oxide stress limit greater than the output voltage of the pumping system. Such a design requires a thick oxide which results in a low performance device. For example, these low performance thick-oxide FETs have a Vt of about 500 mv at worst case, and low voltage pump operation is poor and limited to about 750 mv. Thick-oxide FETs also use VPP-boosted phases which must be distributed to all pump banks and burn C(VPP)2 power. Vds stresses can also be remedied by stacking FETs to share the high differential voltage; however, this is expensive and requires a large amount of chip space.